MPO is the fourth Metal Gear title made specifically for the PSP[3] and the first one to retain the series' action-based play mechanics.[4] It is the first canonical outing of the Metal Gear Solid series for a portable platform, following the storyline of Metal Gear Solid 3: Snake Eater.[5] Set in 1970 in South America, six years after the events of Snake Eater, the game follows the exploits of Naked Snake after his former unit, FOX, goes renegade.[4] The game also chronicles the eventual founding of FOXHOUND and The Patriots, as well as the inspiration of the military state Outer Heaven.[4][6]

Metal Gear Solid: Portable Ops has received highly positive critical reception from critics, and is often viewed as one of the best games released for the PSP.[7] It was praised for its visuals, usage of Metal Gear elements and story, while criticism was directed at the game's control scheme.

The main addition to MPO is the Comrade System.[9] Instead of the solo missions of previous MGS games, MPO goes for a squad-based approach, with Snake having to recruit allies and form a team of trained specialists.[10][11] Before each mission, the player must compose a four-man squad. The squad is then sent into battle.[11][12] Each member of Snake's squad has his or her own strengths and weaknesses.[11][13] While some units are best utilized on the battlefield, others may specialize in producing items, healing allies, or providing intel for each of the game's maps.[12][13]

During missions, the player controls only one squad member at a time.[13] Squad members not in use will hide themselves inside a cardboard box, and can be swapped into play when the player-controlled character finds a hiding spot, where he or she will hide in a cardboard box.

A variety of methods can be employed to expand one's squad.[14] If an enemy character is tranquilized or stunned, they can be dragged to a waiting vehicle and captured.[6][14] After a few in-game days, the captured soldier will become a member of Snake's team.[14] The player can also drag enemy characters to any ally waiting in a cardboard box, where, through the use of a transceiver frequency, or by giving the cardboard box a "nudge", the ally will transport the enemy for the player, saving stamina.[14] Alternatively, by accessing the PSP in certain hotspots using the system's Wi-Fi feature, soldiers and even special bonus characters can be recruited.[10][14] The PSP GPS Receiver can also be used to similar effect.[11][14] Since the player's team consists primarily of former enemy soldiers and personnel, generic characters can walk among their own kind undetected as long as the player avoids suspicious actions such as pointing a gun or being spotted by an enemy of another type.[14] Characters that can be recruited by the player include Soviet soldiers, FOX unit members, high-ranking officers, scientists, engineers and government officials. In addition to the standard male characters, the player can recruit and control female scientists and officers as well. The player can also recruit the GRU, KGB and Ocelot unit soldiers from MGS3, but these are only attainable by AP Scan.

Characters who are killed in combat are eliminated from the player's squad permanently.[11][12] "Unique characters" (i.e. characters important to the game's story, who can only be added into the player's squad by fulfilling certain tasks) are exempt from this rule.[11] If a unique character is wounded in combat, they are sent to an infirmary to recover, making them unusable for a few in-game days.[11] The player can also restart or abort a mission at any moment.[15]

Another new feature is the surround indicator added to the game's HUD.[13][16] Similar to the radar in previous titles, the surround indicator allows players to determine the relative proximity of enemy soldiers by the noises they make.[16] The surround indicator is composed of two circles; the outer circle displays the noises made by enemies and inner circle displays noises made by the player's character.[16][17]

The game contains a Wi-Fi-enabled multiplayer mode, which is an expansion of the "Metal Gear Online" mode previously featured in MeSubsistence.[10][18] One's performance in the Online Mode may affect his performance in the single player campaign; the player can recruit and trade soldiers from beaten opponents, or vice versa.[8][11] Additionally, certain multiplayer options result in recruits being removed from the one's single player roster permanently.[12]

In contrast to the console games in the series, the cutscenes that drive the story are not rendered using the usual real time engine. Instead, they are presented using an animated comic style consisting of hand-drawn artwork by artist Ashley Wood.[10] This style was previously utilized in Metal Gear Solid: Digital Graphic Novel.[4][6] The game also features voice acting, consisting of returning cast members from Snake Eater and new actors.[1][19] However, the number of cutscenes and voiced sections are minimal, due to the PSP's UMD storage capacity constraints.[11]

The main antagonist is Gene (Norio Wakamoto, Steve Blum), the leader of the FOX unit who seeks to establish his own military nation.[21] He is a product of the Successor Project that aimed to artificially create the perfect commander.[22] He is assisted by Lt. Cunningham (Daisuke Gōri, Noah Nelson), an expert in interrogation techniques.[23] (who is later revealed to be a double agent of the Pentagon sent to tarnish the CIA's reputation)[24] Gene also has an adoptive daughter named Elisa (Saori Goto, Tara Strong) who is gifted with extraordinary psychic abilities, implied to be the result of exposure to nuclear fallout during the Kyshtym disaster. Elisa suffers from dissociative identity disorder and has a developed a second personality called Ursula', whose psychic abilities are stronger than her "Elisa" personality. As "Ursula", she works as a member of FOX, while her "Elisa" personality is a medic who takes care of Null and an informant for Snake. Snake meets Elisa, who initially tells Snake that she and Ursula are twin sisters, only to later learn the truth.[1][25] Two FOX members previously associated with Snake include Null (Jun Fukuyama, Larc Spies), a teenage assassin trained to be the perfect soldier.[26] and Python (Yusaku Yara, Dwight Schultz), a former war buddy of Snake who was previously presumed dead during the Vietnam War.[27]

In 1970, six years after the events of Snake Eater, Naked Snake's former team, FOX unit, has broken their allegiance with the CIA and gone rogue. Snake is also targeted by the FOX unit, which has sent renegade FOX unit soldiers to capture him. The game begins with the torture interrogation of Snake by one of the FOX members, Lieutenant Cunningham. Lt. Cunningham is trying to locate the missing half of the Philosopher's Legacy, with the United States Government having already acquired the other half of the Legacy from the Soviet Union at the conclusion of Snake Eater.[28] Snake is imprisoned in a cell next to Roy Campbell, the sole survivor of an American Green Beret team sent in to investigate the base.[29] Snake learns through Roy that they are on the San Hieronymo Peninsula (a Russian transliteration of "San Jerónimo Peninsula") or "La Península de los Muertos", the site of an abandoned Soviet missile silo in Colombia.

The two escape and Snake makes his way to a communications base, where he attempts to contact his old CO, Major Zero. Instead, he is greeted by his old FOX comrades Para-Medic and Sigint, who reveal that Snake and Zero are being charged for treason and that the only way for Snake to be exonerated from the charges is to find and apprehend the leader of the rebellion, Gene. To complicate matters, Gene has also convinced most of the Soviet soldiers stationed at the base to join their side by simply taking over the chain of command belonging to a Soviet unit which was secretly stationed inside the Colombian territory. In order to complete his mission, Snake must persuade enemy soldiers to join his ranks due to the scale of his mission.

Snake and his squad defeat the top members of the FOX unit and eventually they make their way into Gene's guesthouse. Snake learns many things on his way. Cunningham was working for the Pentagon and wanted Snake to push Gene into launching a nuke at the Soviet Union to tarnish the CIA's reputation and to prolong the Cold War. Gene was actually aware of this plan from the beginning due to information from Ocelot. Gene really wanted to launch a nuke at America to destroy the Philosophers and to make his nation of soldiers, "Army's Heaven". Snake destroys an experimental model of the ICBMG (the Metal Gear model) codenamed RAXA and eventually defeats Gene, destroying the finished ICBMG model afterward. After Gene is defeated he gives Snake the funds, equipment, personnel, and all other information regarding "Army's Heaven".[30] On his return home, Snake is awarded for his actions, he then establishes FOXHOUND afterwards. Elsewhere, Ocelot kills the DCI (Director of Central Intelligence) and takes documents containing the identities of the Philosophers in an effort to "end them".[31]

In the post-credits epilogue, Ocelot speaks with an unknown man on the phone, they are plotting to use the Legacy to fulfill their own agenda. Ocelot actually wanted the trajectory data of the nuke to point to the DCI, in order to black mail the DCI into giving Ocelot the documents containing the true identities of the Philosophers. Ocelot agrees to join his new employer's project under the condition that Snake/Big Boss participates as well.[32]

The game was conceptualized when the Kojima Productions staff decided to make the first Metal Gear Solid chapter rather than another spin-off for the PlayStation Portable. Hideo Kojima had the idea of the player being able to recruit comrades with the Wi-Fi play. As a result, the game was specifically designed for the hand-held console rather than the main platform. Most of the staff had previously worked in the spin-off Metal Gear Acid 2 making Portable Ops their first time doing a main installment. Their biggest challenge was adapting the gameplay from the previous games to Portable Ops as the PlayStation Portable lacked a right analog. Since players cannot spin the camera with the PlayStation Portable the game was added a sound indicator system that helps them to see where there are enemies. Impressed with Ashley Wood's work in Metal Gear Solid: Digital Graphic Novel the studio asked his collaboration to illustrate the events accompanied with voice-overs from the story replacing the typical use of cutscenes previously used.[33]

The game was first released in North America on December 5, 2006.[34] In Japan, it was released two weeks later, on December 21, in two limited edition packages, with both of them sharing most of its unique bonus content, such as a special camouflage for the PlayStation Portable, as well as a set of three original lapel pins.[35]

In Europe, the game was set to be released in April, yet it was delayed for a month.[36] In the United Kingdom, the game could only be released on May 25, 2007, after it was revealed that the required BBFC rating was missing, forcing retailers to send back their stocks.[37] The added features for the European release included new maps for the single-player campaign and multiplayer mode, characters, missions, player careers, as well as a new "Boss Rush" mode.[38][39]

On November 1, 2009, the game was released on the PlayStation Network for download on the PSP in all three regions.

Metal Gear Solid: Portable Ops received positive reviews. The game scored an average of 86.95% based on 43 reviews on GameRankings and an 87/100 based on 54 reviews on Metacritic.[45]IGN[11] and GameSpot[8] in particular both awarded the game 9 out of 10.

Metal Gear Solid: Portable Ops Plus (officially abbreviated MPO+) is a stand-alone expansion of the original MPO, focused primarily on online play. It was first announced on July 17, 2007[47] and was released in Japan on September 20, 2007,[47] in North America on November 13, 2007[48] and in Europe on March 28, 2008.[49] A digital version was released on the PlayStation Store in 2009.

MPO+ include new general and unique character types from other MGS titles, as well as new items and weapons, new multiplayer maps, and new game modes.[47][50] While MPO+ does not require the original MPO, players who have save data from the original game can transfer their squad to the expansion and any unique character that the players recruit in the original MPO since their initial save file was created will be added automatically in MPO+ if detected. The following changes have been made to the game.

The player can now recruit up to 200 soldiers. Soldiers now have new careers and skill levels. Players can now obtain textbook items that can raise the stats of their recruits.

The story campaign has been eliminated and a new "Infinity Mission" mode has been added in its place, which has four difficulty settings. Each difficulty consists of a series of randomly-generated stages (except on Easy, in which the stages are already determined) set in locations from the original MPO that the player's team must clear in succession. During certain stages, the player is given a special challenge such as reaching the goal without being seen or survive an alert phase for a certain period. After clearing a special challenge, the player is allow to sort his team and replace any of its members and gear with soldiers or items procured in previous stages. The player is also given a choice to suspend their game and resume from where they left off at a later time. If the player successfully complete every stage in Infinity Mission or uses a Fulton balloon to escape, they will retain every soldier, item and experience points acquired since the mission started (conversely, any item or soldier lost during the mission will be permanently lost as well). However, if the player fails or aborts the mission, the player's squad will be set to the way it was before the mission began (undoing any deaths that occurred as well).

If the game detects save data from the original MPO, a boss rush mode will be unlocked in which a team chosen by the player must face against all the bosses from the original MPO successively. Clearing the boss rush will add any of boss character from the original MPO that hasn't already been added to the player's team, although any team member killed during the process will be lost as well much like in Infinity Mission.

Roy Campbell, who was only playable in the original MPO by hacking the game, can now be recruited as a legitimate playable character. Other new unique characters added to MPO+ include Raiden from Metal Gear Solid 2, Old Snake from Metal Gear Solid 4, and Johnny the Guard from Metal Gear Solid 3. The player can now recruit Elisa and Ursula at the same time as well (the original MPO only allowed the player to have one or the other, since in the game's story they're the same person).

Any unique characters that is killed in action will be eliminated from the team. However, lost unique characters will have a probability of showing up in Infinity Mission as random prisoners, giving the opportunity for the player to recover any of them if possible.

New soldier types can be recruited such as female Soviet soldiers, the Genome Soldiers from the original Metal Gear Solid and various soldier types from MGS2. There are also female members of the Ocelot Unit from MGS3, but these can only be recruited through the game's "AP Scout" feature, in which the player recruit soldiers through LAN access points.

Five new maps have been added to the multiplayer mode, including a recreation of Rex Hangar from the original MGS. The Western Wilderness and Ravine stages previous exclusive to the European version of the original MPO is now available for all regions as well.

^Roy Campbell: Right. But as you can see, they had already completed part of the facility. We were sent in to investigate what was going on. Before we could do that, they ambushed us. // Naked Snake: "They?" // Roy Campbell: The FOX Unit. My team was wiped out in the blink of an eye. I fear I'm the only survivor. (Metal Gear Solid: Portable Ops) Kojima Productions, 2006

^Gene: (...) My goal of building a new nation of mercenaries with only the most capable soldiers is real. (...) (Metal Gear Solid: Portable Ops) Kojima Productions, 2006

^Gene: (...) I was created for that purpose. // Naked Snake: Created? // Gene: Have you heard of the Successor Project? // Naked Snake: ... No. // Gene: It was a top-secret experiment conducted by the U.S. government. Its goal: To create the ultimate battlefield commander. (...) (Metal Gear Solid: Portable Ops) Kojima Productions, 2006

^Cunningham: All right, then, Snake, I'll explain it to you. I don't answer to Gene.Naked Snake: What?Cunningham: The CIA may control FOX, but I don't answer to them, either. My employer is the U.S. Department of Defense.Naked Snake: What do you mean?Cunningham: Everything you've seen so far -- Surely you know that the CIA and the Pentagon aren't on the best of terms?Naked Snake: You're talking about a fight over budget?Cunningham: No, not that. The Bay of Pigs Invasion in 1961... The covert ops in North Vietnam - the influence of the CIA in America's military affairs grows with each passing year. The Pentagon brass started to feel threatened. And then they found out about the Metal Gear project. They enlisted Gene, FOX Unit's commanding officer, and had him steal it before it could be handed over to the Russians. What do you think would happen if Gene were to launch Metal Gear into the Soviet Union? The CIA'd lose face, and the military's influence would immediately start to soar. (Metal Gear Solid: Portable Ops) Kojima Productions, 2006

^Naked Snake: It's... it's not like that... When I saw you at the heliport with Cunningham and the others... // Elisa: That... was my sister. // Naked Snake: Sister? // Elisa: Her name is Ursula. She's the one who's a member of FOX. I'm just a medic. (...) Ursula and I were raised in an East German lab. They sent all the kids with ESP potential there to study them. My powers are pretty weak. But Ursula is different. She's one of the most powerful psychics in all the Communist world. (...) (Metal Gear Solid: Portable Ops) Kojima Productions, 2006

^Naked Snake: So this is what you were calling the Perfect Soldier? Does he have ESP too? // Elisa: No, he's just an ordinary human being. That is, he used to be... He was raised in a special way, under very special conditions: To be a great warrior. He's incapable of emotions or doubts. He's the ultimate combatant, created with one purpose only: To accomplish the mission. He has no human memories. The only things he has left are his skills in battle. (Metal Gear Solid: Portable Ops) Kojima Productions, 2006

^Naked Snake: Python... So you were alive all this time. (...) // Python: (...) How long has it been since our last mission together? Nearly ten years? We were both so very young back then. // Naked Snake: Yeah... When The Boss vanished... and I didn't know what the hell I was doing... you were there to save me. (...) But I thought you were dead. You were wounded on that top-secret mission in Vietnam. (Metal Gear Solid: Portable Ops) Kojima Productions, 2006.

^Kojima Productions. "Metal Gear Solid: Portable Ops". Konami. Cunningham: Simply apply the right type of pain, to the right degree, at just the right location. It's always been my policy for persuasion, and I've found it to be quite effective. The CIA only recovered half of the Legacy from the Soviets after Operation Snake Eater. And you know where the other half went. / Naked Snake: The CIA...only got half? / Cunningham: You can drop the act. You defeated The Boss and returned alive, so you must know where the other half of the Legacy is. And now...you're gonna tell me, Snake. / Naked Snake: Don't have a clue. / Cunningham: That's too bad. Because I'd hate to have to continue persuading you like this, "comrade."

^Kojima Productions. "Metal Gear Solid: Portable Ops". Konami. Campbell: Right. But as you can see, they'd already completed part of the facility. We were sent in to investigate what was going on. Before we could do that, they ambushed us. / Naked Snake: "They"? / Campbell: The FOX Unit. My team was wiped out in the blink of an eye. I fear I'm the only survivor. / Naked Snake: What's the FOX Unit doing in a Soviet base? / Campbell: Not a clue. I'd have expected you to know more about it than me.

^Kojima Productions. "Metal Gear Solid: Portable Ops". Konami. Gene: The equipment, personnel, and funds I amassed in secret to build Army's Heaven. All the data is stored on that film. No one else knows about it. / Naked Snake: Why are you giving me this? / Gene: Because you and I are the same. Some day, you'll be glad you have it... The one who fights and survives must carry on the legacy. Such is our fate. Go, Snake. I've said everything I need to. You are the one who will inherit my genes. You are the true successor. Be loyal to yourself... Go forth. And find your own calling.

^Kojima Productions. "Metal Gear Solid: Portable Ops". Konami. DCI: What the hell are you doing?! You're not planning to betray the Philosophers, are you? / Ocelot: Betray? No, I'm not going to betray them. I'm going to end them! Then we'll take back what you stole from us. We will carry on the spirit of the true patriot. / DCI: The true patriot? / Ocelot: It's all part of our plan to make the world she envisioned a reality. And so I've come for the other half... of the Legacy!

^Kojima Productions. "Metal Gear Solid: Portable Ops". Konami. Ocelot: Yes, the DCI's death should pass as suicide. They'll claim he did it because he felt responsible for the theft of Metal Gear. We can look forward to a major shake-up on the seventh floor at Langley shortly. It looks as if everything is going exactly the way you wanted it. When the DCI saw the trajectory data you supplied and found out that he was the target of the nuclear strike, he brought the Philosophers' documents right to me. Yes, we should have known Gene was serious about launching those nukes... Seems our insurance policy came in handy after all. He did quite well... And now I've got the Legacy. Is that all part of your script, too? Using him and the FOX Unit like that... Only you could have pulled it off. But... You won't be using me anymore ...Battle data... ...from the Perfect Soldier? Genes... genome... I see... Intriguing... I'll help you with the project. But on one condition... I want him to join us. Yes, Big Boss... So that we can become the Patriots.